The objective of the proposed research is to learn more about the mechanisms underlying both normal eye movement control and human ocular motor disorders. The research strategy is to make quantitative measurements of ocular motor function in both human beings and monkeys and to use control systems analysis to better interpret the findings. Our major interest is in the ocular motor functions of the cerebellum with particular emphasis on the mechanisms by which images are stabilized on the fovea for best visual acuity. We will investigate in depth carefully selected groups of patients with disorders of steady ocular fixation (pendular nystagmus and saccadic oscillations). Using both single unit recording and ablation techniques in trained monkeys, we will study the function of the dentate nuclei and perihypoglossal nuclei in the cerebellar control of smooth pursuit eye movements, visual-vestibular interaction and maintenance of steady fixation. Computer techniques and simulations will be used to analyze and interpret data, test models of ocular motor control, and help classify disorders of ocular motility. Our results will provide new information about function and structure and thereby enhance both neuro-ophthalmologic diagnosis and treatment.